Method for processing semiconductor wafer and semiconductor wafer

ABSTRACT

There is provided a method for processing a semiconductor wafer subjected to a chamfering process, a lapping process, an etching process, and a mirror-polishing process, wherein acid etching is performed after alkaline etching as the etching process, and the acid etching is performed with an acid etchant composed of hydrofluoric acid, nitric acid, phosphoric acid, and water, a method for processing a semiconductor wafer subjected to a chamfering process, a surface grinding process, an etching process, and a mirror-polishing process, wherein the etching process is performed as described above, and a method for processing a semiconductor wafer subjected to a flattening process, an etching process, and a mirror-polishing process, wherein the etching process is performed as described above, a back surface polishing process is performed after the acid etching as the mirror-polishing process, and then a front surface polishing process is performed. According to this, there can be provided a method for processing a semiconductor wafer to have good flatness, good surface roughness, and good condition on a back surface thereof.

TECHNICAL FIELD

[0001] The present invention relates to a semiconductor wafer and moreparticularly relates to the improvement of a method to remove amechanically damaged layer on a wafer surface generating in a productionprocess of the single crystal silicon wafer by a chemical etching andthe improvement of a wafer back surface state resulted from a productionmethod of the single crystal silicon wafer.

BACKGROUND ART

[0002] Conventionally, a production process of semiconductormirror-polished wafers usually comprises processes that wafers areobtained by slicing a single crystal ingot made of silicon or the like,and the obtained semiconductor wafers are subjected to at leastchamfering, lapping, etching, mirror-polishing, and cleaning/drying. Anexample of such a production process of semiconductor mirror-polishedwafers is shown in FIG. 3. These processes are performed variouslydepending on the purpose, that is, a part of the process is replaced orrepeated plural times, or processes such as heat treatment, grinding orthe like are added or substituted. For example, in some cases, apolishing process is performed about 3 steps, or a surface grindingprocess is added prior to a polishing process.

[0003] Generally, etching is intended to remove a surface mechanicaldamaged layer introduced in mechanical processing such as slicing,chamfering, lapping, or the like, and an etching process is performedafter a flattening process such as lapping process or the like. Forexample, an usual acid etching process is that several to dozensmicrometers from the wafer surface is etched by a mixed acid aqueoussolution composed of hydrofluoric acid, nitric acid, acetic acid andwater.

[0004] By acid etching, a mechanically damaged layer can be removed, butflatness of a wafer is easily degraded in proportion to increase of itsetching removal. Particularly, an etching amount in a peripheral portionof a wafer is larger than that in the other portion thereof, anddegradation of flatness is remarkable in the peripheral portion. Also,there is a problem that harmful NO_(X) is generated by chemical reactionin acid etching.

[0005] In order to avoid these problems, alkali etching has beensometimes used. However, when etching is performed by using an alkalinetype etching solution as an etchant, the flatness after lapping ismaintained as it is, but pits having a depth of several micrometers anda size of from several to dozens micrometers are easily formed locallyon a wafer surface. It is considered that a large part of localmechanical damages generating in a lapping process is etched deeper thanthe other part due to anisotropy of alkali etching, so that pits areformed thereon.

[0006] As described above, since flatness of a wafer is degraded by acidetching and pits are formed on a wafer by alkaline etching, it isnecessary to increase a stock removal for mirror-polishing to removethose. However, by increasing stock removals, the flatness is degradedby polishing and productivity of a polishing process is decreaseddrastically.

[0007] Thus, as previously disclosed in Japanese Patent Laid-Open No.11-233485, the present applicant suggested a resolution such that in anetching process, acid etching was performed after alkaline etching onthe condition that a etching amount of alkaline etching was larger thanthat of acid etching by a mixed acid aqueous solution composed ofhydrofluoric acid, nitric acid, acetic acid and water. As a result,although the flatness was able to be achieved sufficiently, it was notalways enough reduction of a stock removal to obtain a mirror-polishedwafer (PW). In late years, reduction of a stock removal has become stillmore important in the requirement of further high flatness.

[0008] Therefore, it was considered as a technique that a (surface)grinding process was performed just before a polishing process and thena wafer surface is polished so as to shorten a polishing time and toprevent peripheral sags. However, it was difficult to control a residualgrinding striation and a grinding damage by grinding, and it wasdifficult to control a grinding damage at 3 μm or less.

[0009] Moreover, it became clear that in wafer processing, degradationof brightness (glossiness) on a back surface of a wafer, generation ofwaviness (periodic surface roughness of 2 mm or more) and a stain,so-called a blue stain (hereinafter, merely called as a stain), which iseasily generated in a low resistivity crystal, were easily appeared.Particularly, by conditions of an etching process (for example, when anetching amount was reduced), there was a case that glossiness on a backsurface of the wafer was degraded in about 15-20%.

[0010] In this regard, glossiness of a wafer (back surface) according tothe present invention refers to JIS Z 8741 (a relative-specularglossiness measuring method), and the glossiness was measured with arelative-specular glossmeter (glossmeter SD) designated by thatstandards based on the above method. In other words, the glossiness wasestimated on setting conditions that brightness in a state that nothingis put on an object position was imagined with 0% for the sake ofconvenience, and it was assumed that glossiness of a mirror-polishedsurface was 100%.

[0011] Conventionally, sometimes mirror-polishing has been performed onboth sides of a wafer, but if a back surface thereof is completelymirror-polished (its glossiness is increased around 100%), there is aproblem that particles are easily attached thereto and easily releasedtherefrom again, or a problem of a contact area such as a electrostaticchuck or the like used for adsorbing a wafer, so that it is necessary todecrease glossiness to a certain range. Depending on a device process orthe like, glossiness is preferably about 30-60% generally.

[0012] Generally, each glossiness of both sides of a wafer depends uponan etching process. In order to obtain a wafer having a high flatness,an etching amount is preferably reduced in an etching process. In themethod that acid etching was performed after alkaline etching,conventionally, etching was preferably performed on the condition thatan etching amount of both sides in total in alkaline etching is about10-30 μm, particularly 20 μm and etching amount of both sides in totalin acid etching is about 5-20 μm, particularly 10 μm, and the totaletching amount of both sides in the whole etching process was about30-40 μm. According to this, glossiness can be controlled to about 40%,but sometimes there is a problem that when an etching amount was furtherreduced, glossiness was reduced to 20% or less. Therefore, in the casethat a method for processing a semiconductor wafer that acid etching isperformed after alkaline etching, there was a problem of quality of aback surface thereof.

DISCLOSURE OF THE INVENTION

[0013] The present invention was accomplished in view of such problems,and its main object is to provide a method for processing asemiconductor wafer and a semiconductor wafer processed by the methodwherein a chemical etched wafer (CW) such that while maintainingflatness of a wafer, a mechanical damaged layer is removed, surfaceroughness is improved, particularly, local deep pits are more shallow,and the wafer has a smooth uneven shape and has an etched surfacedifficult to generate particle or contamination is manufactured, and itsstock removal can be reduced in a mirror-polishing process and quality(glossiness, waviness and stain) of a back surface of the wafer can beimproved.

[0014] In order to solve the above-described problems, the invention ofa method for processing a semiconductor wafer according to the presentinvention that the wafer obtained by slicing a single crystal ingot issubjected to at least a chamfering process, a lapping process, anetching process, and a mirror-polishing process, wherein acid etching isperformed after alkaline etching as the etching process, and the acidetching is performed with an acid etchant composed of hydrofluoric acid,nitric acid, phosphoric acid, and water.

[0015] As described above, in the etching process, a wafer after lappingis firstly subjected to alkaline etching, and a mechanical damaged layeris removed while maintaining flatness of the wafer after lapping, andthen acid etching is performed, so that local deep pits remaining afteralkaline etching, surface roughness, and sharp uneven shape can beimproved.

[0016] In this regard, if acid etching is performed with an acid etchantcomposed of hydrofluoric acid, nitric acid, phosphoric acid, and water,waviness (surface roughness with a period of 2 mm or more) can be morelowered and pit depth can be shallower than acid etching with aconventional acid etchant composed of hydrofluoric acid, nitric acid,and acetic acid. Therefore, a stock removal in the polishing process canbe reduced, and productivity in the polishing process can be improved.Moreover, since the stock removal can be reduced, degradation offlatness in the polishing process can be prevented, and thus a waferhaving high flatness can be produced much more easily. It is consideredthat an agent solution of a mixed acid does not easily run into pitsowing to viscosity of phosphoric acid, and therefore an etching rate inpits becomes slower than that on the other plain.

[0017] Also, the feature of the present invention is that a method forprocessing a semiconductor wafer that the wafer obtained by slicing asingle crystal ingot is subjected to at least a chamfering process, asurface grinding process, an etching process, and a mirror-polishingprocess, wherein the surface grinding process is performed prior to theetching process, acid etching is performed after alkaline etching as theetching process, and the acid etching is performed with an acid etchantcomposed of hydrofluoric acid, nitric acid, phosphoric acid, and water.

[0018] And in this case, according to the method of producing asemiconductor described above, it is preferable that a lapping processis further added to the method and the processing is performed by alapping process, a surface grinding process, and an etching process inorder.

[0019] As described above, since in the present invention, aconventional lapping process is completely substituted with a surfacegrinding process, or a lapping process is added and a surface grindingprocess is preformed prior to a etching process, a large mechanicaldamaged layer, which is locally generated, can be reduced remarkably andgeneration of deep pits can be suppressed.

[0020] And, in the surface grinding process, as compared to the lappingprocess, a shape of the wafer can be easily controlled relatively, andeach wafer having the same shape can be stably obtained. Moreover,variation of thickness of wafers can also be suppressed.

[0021] The surface grinding can remove pits remaining on a wafer evenafter the lapped wafer is etched. According to this, it is consideredthat the surface grinding process is performed after the etchingprocess. However, by the surface grinding, a pattern called a grindingstriation remains on the wafer surface. It was found that in order toremove the grinding striation, an etching amount is necessary to beincreased in a polishing process, and therefore flatness of the wafer isdegraded. Accordingly, in the present invention, the surface grindingprocess is performed prior to the etching process. Particularly, in theprocess combining a lapping process, it is preferable that theprocessing is performed by the processes of lapping, surface grinding,and an etching process in order.

[0022] In this regard, it is preferable that a chamfering process isperformed after a surface grinding process. Also, in the case that theprocessing includes a lapping process, it is preferable that thechamfering process is performed prior to the lapping process. This isbecause a lapping slurry is used in the lapping process, and if a waferis not chamfered, there may be the case that the lapping slurry is hardto run into a wafer central portion, and therefore a peripheral portionof the wafer is extremely sagged. Also, in the case that the processingincludes a lapping process and a surface grinding process, it ispreferable that the processing includes a plural number of chamferingprocesses (at least 2 processes). In short, the chamfering process isnot particularly limited, and the chamfering process can be insertedproperly between the processes depending on its purpose, and it ispossible to replace the chamfering process or add chamfering processesbetween plural processes.

[0023] Also, it is possible that both sides of a wafer can be ground ina surface grinding process, and only a front surface (which is apolished surface in a polishing process in the case that one side of thewafer is mirror-polished) of a wafer can be ground. Particularly, in thecase that a lapping process is performed prior to a surface grindingprocess, it is preferable that only one side, which is a polished sidepolished in the polishing process, of a wafer is polished. This isbecause a back surface (which is not ground) of a finishingmirror-polished wafer after polishing has the same glossiness as a backsurface of a conventionally used wafer.

[0024] In the case that a finished mirror-polished wafer is completed tobe a double-side mirror-polished wafer, it is preferable that both sidesof a wafer is grounded in the surface polishing process. In this case, alapping process is not necessarily required. It is preferable that ifthe lapping process is omitted, the number of process is reduced andstock removals of both sides of a wafer is also reduced in a polishingprocess. In short, the side polished in the polishing process issubjected to a surface grinding. However, if there is no standard for aback surface shape of a finishing mirror-polished wafer particularly, awafer having a high flatness can be obtained even by double-sidegrinding in a surface grinding process and single-side polishing in apolishing process.

[0025] Next, in the present invention, a grinding striation generated ina surface grinding process must be removed by an etching processperformed after a surface grinding process. This grinding striation isalso a kind of a mechanical damaged layer and it is considered that thegrinding striation has a damage of about 6 μm. Therefore, there is acase that if a wafer subjected to surface grinding is subjected to analkaline etching, generation of local deep pits generated after alkalineetching is performed after a lapping process can be prevented, but aportion of the grinding striation subjected to a surface grinding isretained or emphasized to make the portion pit-shaped.

[0026] And in order not to generate a pit due to such a grindingstriation even in an etching process, an alkaline etching is combinedwith an acid etching so as to remove damage while maintaining a flatnessof a wafer. Namely, in the etching process, first, after alkalineetching is performed, acid etching is performed. As described above, inthe etching process, a wafer after a surface grinding is subjected tothe alkaline etching first, so that a mechanical damaged layer isremoved while maintaining a flatness of the wafer after the surfacegrinding, and an acid etching is performed, so that a grindingstriation, surface roughness, and sharp and uneven shape remaining afterthe alkaline etching can be improved while maintaining the flatness. Inthis case, it is preferable that the etching amount of the alkalineetching is larger than that of the acid etching.

[0027] And in this case, if the acid etching is performed with anetchant composed of hydrofluoric acid, nitric acid, phosphoric acid, andwater, waviness can be further reduced and pits generated due togrinding striation can be shallow better than the case that acid etchingis performed with a conventionally and generally used acid etchant oftypes of hydrofluoric acid, nitric acid, and acetic acid. Therefore,since a stock removal in the subsequent polishing process can bereduced, productivity in the polishing process can be improved.Moreover, a since stock removal is reduced, degradation of flatness ofthe wafer in the polishing process can be suppressed, so that the waferhaving high flatness can be much easily produced.

[0028] Also, the present invention provides a method for processing asemiconductor wafer that the wafer obtained by slicing a single crystalingot is subjected to at least a flattening process, an etching process,and a mirror-polishing process, wherein the flattening process isperformed as a previous step of the etching process, acid etching isperformed after alkaline etching as the etching process, the acidetching is performed with an acid etchant composed of hydrofluoric acid,nitric acid, phosphoric acid, and water, and a back surface polishingprocess is performed as the mirror-polishing process after the acidetching, and then a front surface polishing process is performed.

[0029] As described above, in the mirror polishing process, since theback surface polishing process is performed after the acid etching andthen the front surface polishing process is performed, quality of theback surface of the wafer (glossiness, waviness, and stain) can beimproved. Also, since the back surface polishing process is performedprior to the front surface polishing process, generation of transferenceof waviness on the back surface of the wafer into the front surfacethereof can be reduced, a stock removal for the surface polishing can befurther reduced, a nanotopography level of unevenness can bedisappeared, degradation of flatness due to polishing can be suppressed,and therefore there is an advantage that a wafer having high flatnesscan be obtained. Moreover, since the stock removal can be reduced,productivity of the polishing process can be extremely improved.

[0030] In this case, the flattening process comprises a lapping processand/or a surface grinding process.

[0031] As described above, according to the processing method of thepresent invention, the flattening process comprises a lapping processand/or a surface grinding process, and in the subsequent etching processand the mirror polishing process, the shape and flatness of the wafercan be maintained and a grinding striation can be effectively removed,so that a wafer can be processed to have high flatness and to improvesurface roughness and waviness.

[0032] In this case, it is preferable that the back surface polishingprocess is performed so that glossiness of the wafer is 35-50%.

[0033] As described above, if the polishing is performed so thatglossiness is 35-50%, generation of particles and a contact area forelectrostatic chuck or the like to adsorb a wafer do not become aproblem.

[0034] Moreover, it is desirable that the surface grinding process isperformed so that a peripheral portion of the wafer becomes thicker.

[0035] By the surface grinding, a wafer can be processed to have highflatness. However, in order to make a polished wafer highly flat, it ispreferable that there is prepared a wafer of which peripheral portion(within a range of about 5 mm from a periphery) is about 0.06 μm thickerthan the other portion in the case of a stock removal having a thicknessof 1 μm in the polishing process. This is because the peripheral portionof the wafer is easy to become thinner than the other portion thereof inthe etching process and polishing process, which are later processesthan the surface grinding process. In the surface grinding process, ashape of the wafer is controlled like such a shape that a peripheralportion of a wafer is thicker than the other portion thereof, so thatthe wafer can be produced stably, and as a result, a wafer having highflatness can be obtained after the polishing. Also, variation ofthickness between wafers can be suppressed.

[0036] In this case, a total etching amount on both sides of the waferin the etching process is 30 μm or less.

[0037] As described above, since a total etching amount of both sides inthe etching process is 30 μm or less, a wafer having high flatness canbe obtained. Particularly, peripheral sags on a wafer due to etching canbe prevented.

[0038] Also, according to the present invention, a stock removal in themirror-polishing process can be 7 μm or less.

[0039] In the mirror polishing process of the present invention, since awafer having little waviness and extremely shallow pits can be obtainedin the above etching process, the stock removal can be reduced extremelydown to 7 μm or less, and a mirror polished wafer having high flatnesscan be obtained, so that productivity of the polishing process can besignificantly improved.

[0040] In this case, in order to obtain a good polishing surface, it ispreferable that a stock removal of the wafer is 2 μm or more.

[0041] It is preferable that in the etching of the present invention, acomposition ratio of the acid etchant in mixing is that theconcentration of hydrofluoric acid is 5-15% by weight and theconcentration of phosphoric acid is 10-40% by weight.

[0042] As described above, in order to make pit depth shallow generateddue to alkaline etching after lapping or grinding to reduce a stockremoval, it is preferable that a composition ratio of the acid etchantin mixing (initial concentration) is that concentration of hydrofluoricacid is 5-15% by weight or less and concentration of phosphoric acid is10-40% by weight. Under the conditions, viscosity of the etchant becomesadequate, so that pit depth can be effectively reduced, there is littleeffect of side reaction between phosphoric acid and hydrofluoric acid,and a silicon surface can be subjected to etching stably.

[0043] And, in this case, it is preferable that silicon is dissolved inthe acid etchant so as to have a silicon concentration of 10 g/L ormore.

[0044] As described above, the meltage of silicon is set in largequantity, an exchange amount of an etchant to restore the etchant to aformer state can be reduced. As a result, concentration of the etchantcan be easily controlled, and the acid etching condition can be stable.Also, quality such as waviness can be improved.

[0045] In this case, it is preferable that a composition ratio of theacid etchant in use is that the concentration of hydrofluoric acid is1-7% by weight and the concentration of phosphoric acid is 18-33% byweight.

[0046] Although it is preferable that the initial concentration(concentration in mixing) is set within the above describedconcentration range, it is preferable that the composition ratio of theacid etchant when a wafer is actually subjected to etching(concentration in use) is that the concentration of hydrofluoric acid is1-7% by weight and the concentration of phosphoric acid is 18-33% byweight. If the etching is performed within such a concentration range,pits become shallow and a wafer having good surface condition can beobtained. Hydrofluoric acid is gradually reduced as the etchingtreatment is repeated, and when a hydrofluoric acid concentration is 1%or less by weight, its etching effect becomes too low. If theconcentration is beyond the above range, all or a part of the chemicalis exchanged, so that the treatment can be stably performed.

[0047] It is preferable that an alkaline etchant used in the alkalineetching according to the present invention is an NaOH aqueous solutionor a KOH aqueous solution.

[0048] If such an etchant is used, etching treatment effect can befurther exerted certainly, a wafer has high flatness, an etching amountcan be controlled relatively easily, and it can be adjusted in low cost.

[0049] Next, as for the semiconductor wafer processed by the methodaccording to the present invention, mechanical damages are removed byalkaline etching while flatness after lapping or grinding is maintained,and then acid etching containing phosphoric acid is performed, so that asemiconductor wafer that etching on a deep pit portion particular to analkaline-etched surface is suppressed, and surface roughness and sharpuneven shape are improved can be obtained. Particularly, a semiconductorwafer of which pit depth and waviness are improved and having furtherflatness can be obtained. Moreover, a semiconductor wafer having goodqualities (glossiness, waviness and stain) on its back surface can alsobe obtained.

[0050] Moreover, a semiconductor wafer according to the presentinvention, which is chemically etched (CW), wherein the maximum value ofa pit depth is 4 μm or less, waviness is 0.05 μm or less, and glossinessis 20-70%.

[0051] As described above, in the present invention, a chemical-etchedsemiconductor wafer having extremely small pit depth can be produced.

[0052] Also, the above-described mirror polishing of 7 μm or less isperformed by using the above CW, there can be provided the semiconductorwafer (PW) of which front surface is mirror-polished, wherein SFQRmax is0.1 μm or less, the maximum value of a pit depth on the surface, otherthan the mirror-polished one, is 4 μm or less, waviness is 0.05 μm orless, and glossiness is 20-70%.

[0053] Since the pit depth of the CW can be lowered, a stock removal canbe significantly reduced when a surface of this wafer is polished. Whena stock removal is increased, degradation of flatness (particularly,sags on a wafer periphery) or the like is easily generated, but if astock removal is reduced, the degradation can be prevented, and a waferhaving extremely high flatness of SFQRmax of 0.1 μm or less can beprovided. Also, glossiness on a back surface of the wafer can becontrolled as in a conventional wafer.

[0054] As explained above, according to the present invention, sincephosphoric acid type mixed acid is used after alkaline etching, etchingon a deep pit portion particular to an alkaline-etched surface issuppressed, a flat surface can be obtained, a stock removal for themirror polishing can be reduced, and the productivity in the polishingprocess can be improved under the favor of high viscosity effect due tothe addition of phosphoric acid. Also, waviness can be improved andflatness after mirror polishing can be drastically improved. Moreover,since a stock removal in mirror polishing is reduced, degradation offlatness in the mirror polishing can be suppressed, so that a waferhaving high flatness can be easily produced.

[0055] Moreover, there can be provided a method for producing asemiconductor wafer and a processed semiconductor wafer, wherein whilemaintaining flatness of the wafer, a mechanical damaged layer isremoved, surface roughness is improved, particularly pit depth generatedthereon becomes further shallow, a chemical-etched wafer having a smoothuneven shape is manufactured, and a stock removal in the mirrorpolishing process is reduced about 4 μm in the whole surface. Therefore,productivity in the mirror polishing and flatness of the wafer can beimproved, and cost cutting in the mirror polishing process andimprovement of its quality can be made.

[0056] Also., glossiness on a back surface of the wafer and surfacestain called blue stain can be prevented.

BRIEF EXPLANATION OF THE DRAWINGS

[0057]FIG. 1 includes flowcharts showing a process of producing asemiconductor mirror-polished wafer processed from a single crystalingot. FIG. 1(a) is a flowchart showing an example of a productionprocess according to the present invention. FIG. 1(b) is a flowchartshowing another production process according to the present invention.

[0058]FIG. 2 is a flowchart showing a method for processing asemiconductor wafer comprising a back surface polishing processaccording to the present invention.

[0059]FIG. 3 is a flowchart showing an example of a conventionalproduction process.

BEST MODE FOR CARRYING OUT THE INVENTION

[0060] Hereinafter, embodiments of the present invention will beexplained specifically in reference to tables and drawings. However, thepresent invention is not limited thereto.

[0061] The present inventors performed various studies on a method ofprocessing a semiconductor wafer, particularly an etching method thatflatness of the wafer after lapping is maintained, and a chemical-etchedwafer having an etched surface difficult to generate particles orcontamination is manufactured. As a result, they conceived that alkalineetching is performed first so that a damaged layer is removed whilemaintaining flatness of a wafer after lapping, and in order to improveremaining deep pits, surface roughness or waviness, acid etching isperformed by using a mixed aqueous solution composed of hydrofluoricacid, nitric acid, and phosphoric acid (sometimes called phosphoric acidtype mixed acid) as an acid etchant, by not using a conventional mixedaqueous solution composed of hydrofluoric acid, nitric acid, and aceticacid (sometimes called acetic acid type mixed acid). Consequently, thepresent invention was accomplished through investigations of processingconditions.

[0062] First, as to basic processing conditions of alkaline etching, forexample, it is sufficient that after a wafer having a diameter of 8inches (200 mm) is subjected to lapping with #1200 lapping abrasivegrains, it is subjected to alkaline etching at a temperature of 85° C.in an NaOH aqueous solution of which concentration is 50%. And, as to anetching removal of the alkaline etching, 10-30 μm on both sides in totalis a proper range. Particularly, as to a condition where depth of localdeep pits is near a minimum value, and TTV and Ra are not much degraded,about 20 μm is preferable.

[0063] In this regard, the local deep pits mean that pits are formed sothat lapping abrasive grains stick in a wafer surface in lapping, andsizes and depths of the pits are increased by alkaline etching.Therefore, a size of lapping abrasive grains used in lapping have aninfluence on the pits. Also, if concentration of alkaline is low, thereis a tendency that the pit depth is increased. On the contrary, ifconcentration of alkaline is high, the pit depth can be shallow.However, for that purpose, it is necessary to increase an etchingremoval, and therefore it is not efficient. Besides, this pit depth canbe determined by the depth of focus of the pit utilizing an opticalmicroscope, and in order to remove the pit, it is necessary to polish itin a mirror polishing process, which is a post-process. Therefore, it isnecessary that an amount of mirror polishing is more than a maximumvolume of such a deep pit depth, and therefore it is desirable that thepit becomes shallow as much as possible.

[0064] In this regard, TTV [Total Thickness Variation] (μm) means anumerical value indicating the difference of thickness between athickest part and a thinnest part on a wafer, and it is a index offlatness of a wafer.

[0065] Also, Ra (μm) means center line average roughness, which is oneof the most often used surface roughness parameter.

[0066] Next, the composition of an acid etchant was investigated andexamined.

[0067] As substitute for a conventionally used acetic acid type mixedacid composed of hydrofluoric acid, nitric acid, and acetic acid, aphosphoric acid type mixed acid composed of hydrofluoric acid, nitricacid, and phosphoric acid was examined.

[0068] Namely, an acid etchant is composed of hydrofluoric acid andnitric acid as main materials capable of improving surface roughness,and phosphoric acid as substitute for a commonly used acetic acid ismixed therein. As for the reason, since phosphoric acid is stable amongstrong acids, it was considered that according to influence of viscosityof phosphoric acid, after phosphoric acid is entered into pits, newsolution is cut off from supplies, so that etching in pits can be moreslower than the other plain, and its tendency was able to be graspedqualitatively.

[0069] Therefore, a mixing ratio of hydrofluoric acid, nitric acid, andphosphoric acid was studied.

[0070] Table 1 shows observed results of mixing ratios of acids, localdeep pit depths of acid-etched wafers, and surface conditions on theoccasion that in order to improve waviness and remove a mechanicaldamaged layer, after a wafer having a diameter of 8 inches was subjectedto lapping of 60 μm on both sides in total with #1200 lapping abrasivegrains, the wafer was subjected to alkaline etching of 20 μm on bothsides in total (hereinafter, sometimes called a wafer to be processed),and the wafer was subjected to acid etching with a mixed acid aqueoussolution composed of hydrofluoric acid, nitric acid, phosphoric acid andwater.

[0071] Test numbers 1-13 represent the cases where each mixing ratio ofhydrofluoric acid, nitric acid, and phosphoric acid was changed, andtest numbers 14 and 15 represent the cases where a conventional mixedaqueous solution composed of hydrofluoric acid, nitric acid, acetic acidwas used.

[0072] After silicon of 11 g/L was dissolved in these etchants tostabilize them, a wafer to be processed was subjected to etching of 10μm on both sides in total, and evaluated in a pit depth and surfaceconditions. TABLE 1 Mixing Ratio (Capacity Ratio) HF HNO₃ H₃PO₄ CH₃COOH(Initial) Concentration in mixing Evaluation Items 50% by 70% by 85% by100% by (% by weight) Pit Depth Surface Test Nos. weight weight weightweight HF HNO₃ H₃PO₄ CH₃COOH H₂O (μm) Condition 1 1 4 4 — 4.3 29.4 42.1— 24.2 3.0 (Note 1) Δ 2 1 3 3 — 5.5 28.6 40.9 — 25.0 3.0 (Note 1) ◯ 3 13 2 — 6.6 34.1 32.5 — 26.8 3.0 ⊚ 4 2 6 3 — 7.3 37.6 26.9 — 28.2 4.4 ⊚ 51 2 2 — 7.9 27.0 38.9 — 26.2 3.2 ⊚ 6 1 3 1 — 8.1 42.1 20.1 — 29.7 4.2 ⊚7 3 6 2 — 11.3 38.9 18.6 — 31.2 4.0 ⊚ 8 1 1 1 — 13.6 23.4 33.5 — 29.53.1 ◯ 9 2 1 2 — 16.3 14.1 40.2 — 29.4 5.4 (Note 2) Δ 10 2 2 1 — 16.929.2 20.9 — 33.0 4.3 Δ 11 3 1 3 — 17.5 10.1 43.1 — 29.3 5.8 (Note 2) Δ12 4 1 4 — 18.1 7.8 44.7 — 29.4 5.6 (Note 2) Δ 13 4 4 1 — 19.3 33.2 11.9— 35.6 5.2 Δ 14 1 2 — 1 11.5 39.4 — 20.8 28.3 7.4 ⊚ 15 1 3 — 2 7.7 39.7— 27.9 24.7 7.1 ⊚

[0073] the pit depth was able to be improved by using phosphoric acid asa substitute for acetic acid. Particularly, when etching was performedwith a mixed aqueous solution containing acetic acid, its pit depth wasabout 7 μm, but when etching was performed with a mixed aqueous solutioncontaining phosphoric acid, its pit depth was about 3-4 μm. Also, it wasconsidered that when the concentration of phosphoric acid was higher,pits became smaller.

[0074] As described above, it can be found that the pit depthconsiderably depends on the effect of phosphoric acid.

[0075] As for the surface condition, in the case like test numbers 9,11, or 12 that the concentration of hydrofluoric acid (% by weight) washigher than that of nitric acid, the surface became rough, and thesurface was clouded, so that its glossiness was degraded. Whenhydrofluoric acid was too thick or nitric acid was too thin, the surfacecondition was degraded. Therefore, it is preferable that etching isperformed under the condition that the concentration of hydrofluoricacid is thinner than that of the nitric acid. Also, like test 1, if theconcentration of hydrofluoric acid was far thinner than that of nitricacid, etching was not able to be performed (little reaction wasoccurred). Therefore, it is preferable that the ratio (hydrofluoricacid/nitric acid) is set 1/7 or more. On such a condition of no etching(little reaction), the condition of the wafer surface is the same as theoriginal wafer surface condition after alkaline etching, so that deeppits and rough condition in surface roughness can be remained. Also, ifetching is continued, pits become lower, but etching time is getting toolong, and therefore, it suffers from a problem of operations. Also, evenif the concentration of phosphoric acid is 40% by weight or more,etching might be slow. It is considered that etching is difficult toproceed due to the influence of side reaction between hydrofluoric acidand phosphoric acid, or the like.

[0076] In consideration of the above findings, a wafer having very goodsurface condition can be obtained under the condition that the ratio(hydrofluoric acid/nitric acid) is set about 1/2, and the concentrationof phosphoric acid is 40% by weight or less. Glossiness can becontrolled within the wide range of 20-70% by controlling etching time.

[0077] At this point, the surface conditions of Table 1 were foundthrough a visual inspection that each surface became rough and eachglossiness was degraded or each condition after alkaline etching wasremained on the surfaces, and when the wafer had the surface conditiondifficult to use as it was, it was regarded as Δ (not good). If thecondition after alkaline etching is remained on the surface, etching isperformed for a long time so that its surface condition can be improved,but there is a difficulty in terms of operations. Also, even if thesurface becomes rough, its pit depth becomes shallow, so that it can beused as a raw material wafer for subjecting double-side polishing. Eachsurface is subjected to etching, and when the surfaces have goodglossiness or have few spots, they are regarded as ◯ (good). They can beused with no particular problem. Moreover, when each glossiness of thesurfaces is improved, the surfaces have the same condition as thesurfaces subjected to acid etching with a conventional acetic acid typemixed acid, and its glossiness can be controlled within the desiredrange required as products, they are regarded as ⊚ (very good). Also,the pit depth is determined by scanning a wafer surface and by the depthof focus of scanned pits utilizing an optical microscope, and maximumvalues of obtained pit depth are shown.

[0078] In view of each pit depth and surface condition, the preferableconcentration range of phosphoric acid type mixed acid as a initialconcentration in mixing is hydrofluoric acid (HF) of 5-15% by weight,nitric acid (HNO₃) of 20-45% by weight, phosphoric acid (H₃PO₄) of10-40% by weight, and water (H₂O) as the retained material.

[0079] This is because when the concentration of hydrofluoric acid is 5%by weight or less in mixing, etching effect (reactivity) is degraded.Also, when the concentration of hydrofluoric acid is 15% by weight ormore, its surface condition is degraded in relation to nitric acid. Asfor the nitric acid, although any definite findings can not be obtained,it is preferable that the concentration range of the nitric acid is20-45% by weight in relation to hydrofluoric acid and phosphoric acid.Particularly, it is preferable that the ratio (HF/HNO₃) is 1/(2-7). Asfor the phosphoric acid, when its concentration is 10% by weight orless, improvement effect for pits becomes lower, and when itsconcentration is 40% by weight or more, side reaction with hydrofluoricacid, water, or the like is increased, etching becomes unstable, andfinally etching effect wears off, so that the surface condition may bedegraded.

[0080] As for the etching removal of the acid etching, the proper rangeis 5-20 μm on both sides in total. Particularly, when the etchingremoval is about 10 μm on both sides in total, the surface can be etchedsmoothly while pits can become shallow while maintaining its flatness.It is considered that since a mixed acid is difficult to enter into pitsgenerated due to lapping or pits in a grinding striation portionaccording to influence of viscosity of phosphoric acid, etching speed inthe pits is slower than that of the other plain.

[0081] Next, each composition ratio during actual etching was confirmed.In mixing, the above concentration range is preferable, but thecomposition varies during actual etching. When silicon wafers areetched, each composition in the chemical has each tendency that theconcentration of hydrofluoric acid decreases, the concentration ofnitric acid gradually decreases, the concentration of phosphoric aciddoes not change very much, and the concentration of water increases.Particularly, each composition was unstable immediately after etchingwas started.

[0082] In this regard, it is preferable that silicon is previouslydissolved in the mixed acid to stabilize the etchant. The compositionwhen silicon is dissolved therein (the composition when the etchant isactually used) was confirmed.

[0083] Table 2 shows each component's concentration in each etchant whenthe meltage of silicon varied from 0 to 20 g/L. As to an (initial)etchant composition in mixing, the phosphoric acid type acid, which isthe same composition as that of test number 13, i.e., hydrofluoric acidof 50% by weight, nitric acid of 70% by weight, and phosphoric acid of85% by weight are blended in a volume ratio of 1:3:2, respectively, andit was confirmed how did actual etching composition vary when siliconwas dissolved therein.

[0084] Also, since it was conformed that waviness and peripheral sagswere able to be extremely improved by meltage of silicon, each wavinesswas determined by using each etchant shown in Table 2, and a wafer wassubjected to lapping with #1200 abrasive grains, and the wafer, whichwas subjected to alkaline etching of 20 μm on both sides in total, wassubjected to acid etching of 10 μm on both sides in total to estimateeach waviness. These results are also shown in Table 2. TABLE 2Composition of etchant after silicon is dissolved Other HF HNO₃ H₃PO₄H₂O by-product Silicon Meltage (% by (% by (% by (% by such as Waviness(g/L) weight) weight) weight) weight) H₃SiF₆ (μm) 1 0 6.6 34.1 32.5 26.9— 0.104 2 5 5.3 32.8 32.7 27.6 1.6 0.065 3 10 4.0 31.6 32.9 28.3 3.20.044 4 15 2.6 30.2 33.1 29.1 5.0 0.034 5 20 1.2 29.0 32.6 30.6 6.60.029

[0085] As seen from Table 2, when silicon is previously dissolved in theetchant, its concentration in mixing (initial concentration) varies. Byusing a phosphoric acid type mixed acid that hydrofluoric acid of 50% byweight, nitric acid of 70% by weight, and phosphoric acid of 85% byweight are blended in a volume ratio of 1:3:2, respectively, thecomposition of the etchant in use is that hydrofluoric acid is about1-7% by weight, nitric acid is 25-33% by weight, and phosphoric acid is18-33% by weight, and the etchant having such a composition range can bepreferably used. Also, it is more preferable that if silicon of 10 g/Lor more is dissolved therein, a very good wafer having a waviness of0.05 μm or less can be produced. The more the meltage of the silicon isabundant, the more waviness can be reduced.

[0086] In this regard, Waviness is defined that a start point ofmeasurement and an end point of measurement that are determined to havethe same height is assumed as the origin in the height direction, theabsolute values Y₁ to Y₂₉ of displacement from the origin are measuredat intervals of 2 mm, and the average Y of the absolute values Y₁ to Y₂₉represents waviness.

[0087] Waviness was measured by use of the universal surface shapemeasuring device (Type: SE-3F, product of Kosaka Laboratory Ltd.).Specifically, a central area of the surface of a wafer (diameter: 200mm) was traced for 60 mm through use of a stylus within in order tomeasure the surface shape excluding the component of fine surfaceroughness.

[0088] Also, when silicon of 10 g/L is dissolved in an etchant, thecomposition of the etchant can be stable. An etchant right after mixingis unstable in reactivity. If a certain degree of silicon is previouslydissolved in an etchant, reactivity and composition of the etchant canbe stable. Also, in order to bring back to the concentration of theetching composition as silicon is not dissolved therein and right afterthe acids are mixed, it is necessary to exchange almost all the etchant.However, it is easy to bring back to the state of the etchant aftersilicon is dissolved therein, and it is sufficient that only the etchantthat silicon is not dissolved is partially exchanged (added) thereto, sothat an exchange volume of the etchant can be reduced. As the result,the concentration of the etchant and its variation can be lowered andeasily controlled, so that the condition of acid etching can be stable.

[0089] As described above, it was found that using the mixed acidcontaining phosphoric acid for acid etching is superior to using aconventional acetic acid type etching in terms of the following points.

[0090] That is,

[0091] 1) Pit depth can be shallow better than conventional etching ofalkaline etching+acetic acid type mixed acid etching.

[0092] 2) Efficiency of flatness becomes high.

[0093] 3) Waviness component is few.

[0094] 4) Surface roughness becomes fine and glossiness is increased.

[0095] Each etching method, quality of obtained wafers, and eachcharacteristic are arranged in Table 3 and compared. In this regard,when etching was performed only with a phosphoric acid type acidsolution, there was a tendency that flatness was degraded when its stockremoval was increased, as in using only an acetic acid type acidsolution. Also, since an etching speed was slow, its productivity wasnot good. It is preferable that since a wafer is treated with theetchant (alkaline+phosphoric acid type) as the present invention, theetching removal treated with phosphoric acid type etchant can bereduced, above problems are improved, and a stock removal in thepolishing process can be extremely reduced. According to the above, itis clear as to the advantage of the present invention. TABLE 3 Qualitiesof wafer PW Back surface Kinds of etchant Flatness Waviness stockremoval condition Alkaline solution ⊚ (Good) ⊚ (Small) X (Large) X(Rough) (NOH or KOH) Acid solution X (Bad) X (Big) ◯ (Small) ⊚ (Smooth)(Acetic acid type) Combination ◯ ◯ Δ Δ (Alkaline + Acetic acid type)Present invention ◯ ⊚ ⊚ ◯ (Alkaline + Phosphoric acid type)

[0096] In this regard, if an etching removal (stock removal by etching)is too large, peripheral sags on a wafer are easily generated even by anetching of alkaline etching and acid etching. Therefore, in the etchingprocess, it is preferable that the etching amount is set at about 10-30μm on both sides in total that is smaller than a conventional stockremoval. Preferably, it is set at 15 μm on both sides in total foralkaline etching and about 5 μm on both sides in total for acid etching.

[0097] According to the above-described two-stage chemical etching ofalkaline etching+phosphoric acid type mixed acid etching of the presentinvention, a semiconductor wafer having the maximum value of pit depthof 4 μm or less, PV value of 0.05 μm or less of which waviness is at apitch of 2 mm, and glossiness range of 20-70% can be easily producedstably.

[0098] The present inventors also performed various studies on a methodfor producing a wafer having less mechanical damages, pits generated dueto the damages or the like before a mirror-polishing process,particularly an etching process, and its previous process, and as theresult, it was conceived that by incorporating a surface grindingprocess as substitute for a lapping process or after a lapping process,a damaged layer generated due to slicing or lapping is extremelyreduced, and a wafer maintaining a high flatness is obtained, andmoreover, the wafer is subjected to alkaline etching to remove a damagedlayer and grinding striation remaining in the surface grinding, and inorder to improve pits due to remaining grinding striation, acid etchingis performed with a mixed aqueous solution composed of hydrofluoricacid, nitric acid and phosphoric acid as an acid etchant, and thepresent invention was completed through investigations of processingconditions.

[0099]FIG. 1 includes series of flowcharts of processing a singlecrystal ingot to produce a semiconductor mirror-polished wafer. FIG.1(a) is a flowchart showing a surface grinding process and an etchingprocess in order. FIG. 1(b) is a flowchart showing a surface grindingprocess that a lapping process is added in front of the surface grindingprocess, and an etching process in order.

[0100]FIG. 1(a) shows a process for producing a mirror-polished waferthat a lapping process is completely substituted with a surface grindingprocess. A wafer is obtained by slicing a single crystal ingot in aslicing process, and the wafer is subjected to surface-grinding in asurface grinding process to improve its flatness and to remove amechanical damaged layer generated in the slicing process. Next, thewafer is subjected to chamfering processing in a chamfering process, andthe process enters upon an etching process. In the etching, alkalineetching is firstly performed to remove or shallow a damaged layer and agrinding striation, and then acid etching is performed with phosphoricacid type mixed acid to make a grinding striation shallow. On theoccasion, it is preferable that an etching removal of the alkalineetching is larger than that of the acid etching. Subsequently, mirrorpolishing is performed in a mirror-polishing process, and cleaning anddrying are performed in a cleaning/drying process to manufacture amirror-polished wafer having high flatness.

[0101]FIG. 1(b) shows a process for producing a mirror polished wafer,wherein a lapping process is added in front of the surface grindingprocess. A wafer is obtained by slicing a single crystal ingot in aslicing process, and after the wafer is subjected to rough chamfering ina first chamfering process, it is lapped in a lapping process to improveits flatness and to remove a mechanical damaged layer generated in theslicing process. Subsequently, a surface grinding is performed in asurface grinding process to further improve the flatness. Next, finishchamfering processing is performed in a second chamfering process, andthe process enters upon an etching process. In the etching, alkalineetching is firstly performed to remove a damaged layer and a grindingstriation, and then acid etching is performed with phosphoric acid typemixed acid to shallow a grinding striation. On the occasion, it ispreferable that an etching removal of the alkaline etching is largerthan that of the acid etching. Subsequently, mirror polishing isperformed in a mirror-polishing process, and cleaning and drying areperformed in a cleaning/drying process to manufacture a mirror-polishedwafer having high flatness.

[0102] First, as to a standard condition of the surface grinding, it ispreferable that the rotational speed of a spindle is 4000-7000 rpm, therotational speed of the wafer is 5-9 rpm (in processing) and 3-7 rpm (inspark out), and the feed rate of grinding stones is 0.1-0.3 μm/sec.

[0103] It is preferable that a grinding stone used therein is highYoung's modulus type and a surface grinding apparatus is an in-feed typesurface grinding apparatus of which grinding stone is fed from itscenter.

[0104] A surface grinding apparatus available is a duplex grindingapparatus, which can grind both surfaces of a wafer, or a grindingapparatus, which grind one side of a wafer at a time or only one sidethereof. However, the present invention is not limited to the shape ofthe apparatus, particularly.

[0105] In order to improve flatness and to remove a mechanical damagedlayer, generally, it is sufficient that grinding is performed at 40-60μm on both sides in total (20-30 μm on one side).

[0106] In this regard, in the lapping process, under the influence oflapping abrasive grains, a local deep damage (pit) are formed, but thesurface grinding can process a wafer with few local mechanical damage.

[0107] However, under some influence of conditions of the surfacegrinding process, a grinding striation is still remained by surfacegrinding. This grinding striation is that marks of grinding stones beingfed are remained as lines on a wafer surface.

[0108] Therefore, the etching as aforementioned is performed. First,alkaline etching is performed to remove a damaged layer and a grindingstriation, and acid etching is performed with phosphoric acid type mixedacid to shallow a grinding striation. Subsequently, mirror polishing isperformed in a mirror polishing process, and after cleaning and dryingare performed in a cleaning/drying process, a mirror polished waferhaving high flatness can be manufactured.

[0109] The wafer produced by a series of the above process has veryshallow or no pit and few constituents of waviness. Its surfaceroughness is also small and glossiness is increased. Accordingly, apolishing allowance (stock removal) in a polishing process can beextremely reduced, and therefore a wafer having high flatness can beobtained with high productivity.

[0110] On the other hand, as aforementioned, in a wafer processingprocess, there may be the case that luminance (glossiness) of a backsurface of a wafer is degraded, waviness is generated, and contaminationcalled blue stain (hereinafter, maybe simply called stain), which iseasily generated in a low resistivity crystal is generated. Depending onconditions in the etching process (in the case that an etching removalis reduced, for example), glossiness of a back surface of a wafer isdegraded at about 15-20%.

[0111] Accordingly, in order to solve this problem, the presentinventors were conceived that in a mirror polishing process, after theaforementioned acid etching is performed, a back surface polishingprocess is performed, and then a front surface polishing process isperformed.

[0112] The following is the explanation of the case of performing a backsurface polishing according to the present invention. FIG. 2 is a seriesof a flowchart showing an example of a method for processing asemiconductor wafer having a back surface polishing process according tothe present invention. The feature of this method is that in addition tothe method for processing a semiconductor wafer shown in FIG. 1 asaforementioned, a back surface polishing process is performed prior to afront surface polishing process that a wafer front surface ismirror-polished.

[0113] In a slicing process, a conventional method and apparatus can beused. For example, it is preferable that by using a slicing apparatus ofa wire saw or an inner diameter slicer, slicing is performed so that awarp should be reduced. Particularly, slicing is performed so that awarp (bending) is reduced at 10 p m or less.

[0114] In a chamfering process, a conventional method and apparatus canbe also used. It is preferable that in the early stage after slicing, abeveling process (chamfering process) is performed for rounding an edgeof a wafer periphery to prevent chip or crack, and then a chamferingprocess that a chamfered portion is mirror-polished is performed beforea surface grinding process.

[0115] Particularly, it is preferable that the chamfering process isperformed after the surface grinding process. If the lapping process isperformed in the process, it is preferable that the chamfering processis inserted prior to the lapping process. Needless to say, a pluralnumber of the chamfering processes can be inserted therein. In thepresent proposal, an example such that a first chamfering process isinserted prior to lapping, a second chamfering process is inserted aftersurface grinding, and a mirror chamfering process is inserted after backsurface polishing is shown.

[0116] It is sufficient that the flattening process comprises only thelapping process or only the surface grinding process. However, asaforementioned, the surface grinding process is preferable in that thegeneration of the mechanical damage can be more reduced, and as comparedto the lapping process, the shape of a wafer can be easily controlledrelatively and a wafer having the same shape can be obtained stably. Itis also sufficient that in addition to the surface grinding processafter the lapping process, the processes can be performed in the orderof the lapping process, the surface grinding process, and the etchingprocess.

[0117] In this regard, deep pits can be removed in the surface grindingprocess. Therefore, after the etching process, it can be considered thatthe surface grinding process is inserted prior to the polishing process,but a pattern called a grinding striation is remained in the surfacegrinding process. Also, a damage generated due to grinding is alsoremained about several micrometers. Therefore, in order to remove thisgrinding striation, it is necessary to increase a stock removal in thepolishing process. Therefore, in the present invention, the surfacegrinding process is performed prior to the etching process.Particularly, it is preferable that the processes, which still comprisesthe lapping process, are performed in the order of the lapping process,the surface grinding process, and the etching process. According tothis, deep pits generated due to lapping can be removed and a grindingstriation generated due to surface grinding can be removed or can beshallow.

[0118] As to the conditions of a flattening process, in a lappingprocess, it is preferable that the flattening is performed with about#1200-#1500 lapping abrasive grains. It is sufficient that a lappingamount (stock removal for lapping) is about 40-60 μm on both sides intotal. Preferably, as a two-step lapping, a lapping is performed at 40μm on both sides in total with #1200 lapping abrasive grains and theother lapping is further performed at 20 μm on both sides in total with#1500 lapping abrasive grains. As described above, it is preferable thata lapping slurry containing fine abrasive grains of #1500 is used in thelatter process.

[0119] In the case of the surface grinding process followed by thelapping, it is preferable that grinding stones having abrasive grains ofabout #1500-#4000 are used. It is sufficient that a grinding amount(stock removal for the surface grinding) is about 10 μm on one side. Itis preferable that a grinding stone used in the surface grinding arehigh Young's modulus type and grinding apparatus is an in-feed typegrinding apparatus of which grinding stones are fed from its center.

[0120] If surface grinding is performed as the flattening process, agrinding striation is still remained on a wafer even after the grindingprocess that damages are hard to be generated, for example. It isconsidered that the grinding striation is a kind of a mechanical damagedlayer, and there is some deformation (damage) therein. Therefore, whenthe surface-ground wafer is subjected to alkaline etching, local pitsexisting when alkaline etching is performed after the lapping processcan be prevented, but there may be the case that the portion on which agrinding striation exists, which is subjected to surface grinding, isretained or emphasized to become pits.

[0121] Conventionally, a surface grinding was performed prior topolishing to obtain a wafer having high flatness, but there was aproblem that a grinding striation was still remained thereon. Accordingto the present invention, if the surface grinding process is performedprior to front surface polishing, etching and polishing can be performedto remove a grinding striation while maintaining its shape.

[0122] Namely, in the etching process of the present invention, asaforementioned, after alkaline etching is firstly performed in theetching process, acid etching is performed. Moreover, in the acidetching, an acid etchant composed of hydrofluoric acid, nitric acid,phosphoric acid and water is used, so that waviness of a wafer can besmall and the grinding striation can be shallow.

[0123] A back surface polishing process is performed after acid etchingand prior to a front surface polishing process. Only adjustment ofglossiness on a back surface thereof can be performed in any processesafter etching (for example, it can be performed after the front surfacepolishing process), but if the back surface polishing process isperformed after acid etching and prior to the front surface polishingprocess, the front surface polishing can be performed on the conditionthat the constituent of waviness on a wafer back surface is improved bythe back surface polishing, so that the influence of transference of aback surface shape can be lowered utmost, a nanotopography level ofunevenness can be reduced, and the front surface can be polished whilemaintaining its condition of high flatness. And, according to this,since not only adjustment of glossiness on the wafer back surface butalso removal of a blue stain can be performed, a wafer having goodquality on its back surface can be produced. It is preferable thatglossiness on the back surface is 35-50%. For that purpose, it ispreferable that a polishing amount (stock removal for polishing) is 0.4μm or less and 0.05 μm or more, particularly, from about 0.1 μm to 0.3μm.

[0124] The front surface polishing process is not particularly limitedto any conditions in which peripheral sags are not generated by thepolishing. However, if the polishing is performed particularly inmultiple steps, it is preferable that a stock removal on an entire frontsurface is 4 μm or less. However, in order to obtain a goodmirror-finished surface, the polishing is desirably performed about 1μm.

[0125] As described above, by reducing the stock removal, a wafer havinghigh flatness while maintaining flatness at the former processes can beproduced.

[0126] Particularly, in the front surface (mirror) polishing process ofthe present invention, since a wafer having small waviness and a veryshallow grinding striation (pit generated due to a grinding striation)is obtained, a stock removal can be extremely reduced and amirror-polished wafer having high flatness can be obtained. Also, sincethe back surface polishing process is inserted prior to polishing, thetransference of waviness on a back surface of the wafer into a frontsurface thereof can be lowered and a stock removal in front surfacepolishing can be further reduced. As the result, there are advantagesthat a nanotopography level of unevenness can be also reduced, thedegradation of flatness due to polishing can be suppressed, and a waferhaving high flatness can be obtained. Also, since a stock removal can bereduced, the productivity in the polishing process can be extremelyimproved.

[0127] Hereinafter, the present invention will be explained morespecifically in reference to the examples and comparative examplesbelow, but the present invention is not limited thereto.

EXAMPLE 1

[0128] As the result of the above consideration, in view of the pitdepth, the surface condition, the etching rate, and the stability of theetchant, it was found that an etchant of which mixing ratio ishydrofluoric acid of 50% by weight:nitric acid of 70% byweight:phosphoric acid of 85% by weight=1:3:2, that is, an etchant ofHF=6.6% by weight, HNO₃=34.0% by weight, H₃PO₄=32.5% by weight, theretained material of H₂O (26.9% by weight), and the meltage ofsilicon=19 g/L, was preferable. The following is the explanation ofexamples using this acid etchant.

[0129] The following etching treatment was performed for lapped wafers(lapping abrasive grain size: #1200) having a diameter of 200 mm (8inches). TTV of this lapped wafer was about 0.8 μm.

[0130] First, the wafers were immersed in an NaOH aqueous solutionhaving a concentration of 50% by weight at 85° C. for 450 seconds inorder to perform alkali etching with a target etching amount being setto 20 μm on both sides in total. Subsequently, the wafers were immersedinto a hydrogen peroxide solution of 0.3% in order to make the surfaceof the wafers hydrophilic. Finally, the wafers were subjected to acidetching with the above mixed acid of hydrofluoric acid of 50% byweight:nitric acid of 70% by weight:phosphoric acid of 85% byweight=1:3:2 (volume ratio) having a liquid temperature of 25° C. with atarget etching amount being set to 10 μm on both sides in total. Theetched wafers were measured for flatness (TTV), surface roughness (Ra),waviness, pit depth, and glossiness in order to evaluate the effect ofetching. The results are shown in Table 4.

COMPARATIVE EXAMPLE 1

[0131] Acid etching was performed under the same conditions as Example 1except that a conventional acetic acid type mixed acid of hydrofluoricacid of 50% by weight:nitric acid of 70% by weight:acetic acid of 100%by weight=1:2:1 (volume ratio) was used as an acid etchant. The resultsare also shown in Table 4. TABLE 4 Qualities of wafers (CW) afteralkaline etching → acid etching The number of Pit processed TTV RaWaviness depth Glossiness wafer (μm) (μm) (μm) (μm) (%) Example 1 1500.92 0.16 0.029 3.2 40 Comparative 50 1.01 0.23 0.034 6.0 36 Example 1

[0132] According to Table 4, wafers having very good flatness (TTV) andwaviness were able to be obtained in Example 1 (by using phosphoric acidtype mixed acid). Also, wafers having very small pit depth were able tobe produced. Glossiness was equivalent to a conventional value, and itwas found that glossiness was able to be adjusted within a standardrange. Also, chamfered portions of the wafers had good surface roughness(Ra), and there is the effect of reduction of loads such as mirrorfinished chamfering.

[0133] Next, CW surfaces in the above Example 1 and Comparative Example1 were polished. Each stock removal was set to 7 μm (target amount) onthe entire front surface. As to polishing conditions, polishing wasperformed by using a polishing apparatus, which is a single waferprocessing type polishing apparatus, a polishing pad, which is anonwoven fabric type polishing pad, and a polishing agent, which is acolloidal silica polishing agent (pH=10.5).

[0134] Each TTV and SFQRmax of the polished wafers were measured, andeach external appearance was inspected.

[0135] In this regard, SFQR (Site Front least-sQuares Range) means thevalue that an average plain of a surface standard as to flatness iscalculated in every site, and the maximum range of unevenness withrespect to the surface is expressed. SFQRmax means the maximum valueamong SFQR values of all the sites on the wafer.

[0136] The results were shown in Table 5. Also, as for ComparativeExample 1, polishing was further performed until these abnormal externalappearances were not appeared (Comparative Example 1b). TABLE 5Qualities of wafers (PW) after polishing The number of Stock processedTTV SFQRmax External removal wafer (μm) (μm) appearance (μm) Example 1150 0.68 0.095 Nothing 6.97 particular Comparative 50 0.70 0.125 Therewere 6.95 Example 1 remaining pits Comparative 50 0.78 0.161 Nothing9.90 Example 1 b particular

[0137] Flatness TTV and SFQRmax were measured by using a flatnessmeasuring device (U/G9500, U/S9600) made by ADE Corporation. Surfaceroughness (Ra) was measured by use of a universal surface shapemeasuring device (Type: SE-3C) made by Kosaka Laboratory Ltd.

[0138] Also, each flatness SFQRmax was measured by use of a flatnessmeasuring device made by ADE Corporation and evaluated in areas of 20mm×20 mm. As to appearances, the existence of pits was confirmed with amicroscope.

[0139] As clear from the above results, in Example 1, a wafer havinghigh flatness and no bad condition in appearance (pits) was able to beproduced by polishing of which stock removal was 7 μm or less. This wasable to be accomplished so that pit depth after etching was lowered lessthan conventional one by alkaline etching and acid etching containingphosphoric acid. Also, since the stock removal was able to be reduced,the polishing efficiency was able to be improved, peripheral sagsgenerated due to polishing were able to be prevented, so that a waferhaving good flatness on a wafer periphery was able to be produced.

[0140] As described above, the CW is subjected to alkaline etching andacid etching containing phosphoric acid, so that its flatness and pitdepth can be improved. Also, a wafer having less waviness can beobtained. Glossiness of both the Comparative Example and the Example canbe controlled equal to each other.

[0141] As for the PW, it can be seen that since there is abnormalappearance in the Comparative Example, a stock removal is not enough.Generally, in order to completely remove pits after etching, it isnecessary to excessively polish a wafer about pit depth+threemicrometers. If a polishing amount is increased, its processing time isalso increased. Also, as seen from the Comparative Example, SFQRmax isdegraded when a stock removal is increased. It is preferable that astock removal is small. In the acid etching containing phosphoric acid,the stock removal was able to be reduced, and a mirror-polished waferhaving good flatness such as SFQRmax was able to be produced.

EXAMPLE 2

[0142] Mirror-polished wafers were produced in the process as shown inFIG. 1(b), the wafers were prepared by slicing an ingot having adiameter of about 200 mm, the sliced wafers were subjected to firstchamfering, then the wafers were lapped by using a lap slurry (lappingabrasive grain size: #1200), and by using the wafer (called lappedwafer), surface grinding was performed on the following conditions.

[0143] Surface grinding was performed by using an in-feed typesingle-side grinding apparatus, a spindle rotational speed was 5500 rpm,a wafer rotational speed was 7 rpm, and the feed rate of a grindingstone was 0.2 μm/sec. Then after, second chamfering was performed.

[0144] After this surface grinding, flatness (TTV) of the wafer wasabout 0.6 μm.

[0145] Next, as to alkaline etching, the wafers were immersed in an NaOHaqueous solution having a concentration of 50% by weight at 85° C. for450 seconds in order to perform alkali etching with a target etchingamount being set to 20 μm on both sides in total. Subsequently, thewafers were immersed into a hydrogen peroxide solution of 0.3% in orderto make the surface of the wafers hydrophilic. Finally, the wafers wereimmersed in a mixed acid of hydrofluoric acid of 50% by weight:nitricacid of 70% by weight:phosphoric acid of 85% by weight=1:3:2 (volumeratio) having a liquid temperature of 25° C. in order to perform acidetching with a target etching amount being set to 10 μm on both sides intotal.

[0146] The acid-etched wafers were measured for flatness (TTV), surfaceroughness (Ra), waviness, visual inspection (and pit depth), andglossiness on each back surface of the wafers in order to evaluate theeffect of surface grinding+etching.

[0147] TTV was measured by using a flatness measuring device (U/G9500and U/S9600) made by ADE Corporation. Ra was measured by using theuniversal surface shape measuring device (Type: SE-3C) made by KosakaLaboratory Ltd., and measurement was performed on a central area of thesurface of the wafers.

[0148] Waviness is defined such that the vertical position of a startpoint and an end point of measurement that are determined to have thesame height is assumed as the origin in the height direction, theabsolute values Y₁ to Y₂₉ of displacement from the origin are measuredat intervals of 2 mm, and the average of the absolute values Y₁ to Y₂₉represents waviness. Waviness was measured by using the universalsurface shape measuring device (Type: SE-3F) made by Kosaka LaboratoryLtd. Specifically, as for measurement, each central area of the waferswas traced for 60 mm through use of a stylus within in order to measurethe surface shape excluding the component of fine surface roughness.

[0149] A visual inspection was performed by observing the presence ofpits by a microscope.

[0150] If a pit was observed, its pit depth was confirmed. The pit depthwas obtained by the depth of focus of a microscope. The pit depth wasrepresented by the maximum value of estimated wafers.

[0151] Glossiness was obtained by a gloss meter SD made by TOYO SEIKIKOGYO Co., LTD. in terms of wafer back surfaces.

[0152] Above results were shown in Table 6. TABLE 6 Item The number PitExample of processed TTV Ra Waviness depth Glossiness Nos. wafers (μm)(μm) (μm) (μm) (%) Example 2 28 0.60 0.12 0.051 1.0 35 Comparative 501.01 0.23 0.060 6.0 36 Example 2

[0153] Next, the above-described etched wafers were mirror-polished. Thesurfaces, which were previously subjected to surface grinding, werepolished, and the target amount of each stock removal was 4 μm.

[0154] As to polishing conditions, polishing was performed by using apolishing apparatus, which is a single wafer processing polishingapparatus, a polishing pad, which is a nonwoven fabric type polishingpad, and a polishing agent, which is a colloidal silica polishing agent(pH=10.5).

[0155] Each TTV and SFQRmax of polished wafers were measured andsubjected to a visual inspection.

[0156] SFQR was measured by using the flatness measuring device made byADE Corporation, and a size of each site was in an area of 20 mm×20 mm.

[0157] Results are shown in Table 7. TABLE 7 The number Item ofprocessed TTV SFQRmax Visual Stock removal Example Nos. wafers (μm) (μm)Inspection (μm) Example 2 28 0.71 0.120 Nothing 4.00 particularComparative 50 0.70 0.125 There were 4.00 Example 2 - a remaining pitsComparative 50 0.78 0.161 Nothing 9.90 Example 2 - b particular

[0158] As seen from the above results, in Example 2, by polishing ofwhich stock removal was 4 μm or less, a wafer having high flatness butno abnormal appearance (grinding striation or pit) was able to beproduced. This was able to be accomplished, because before etching,particularly before alkaline etching, surface grinding was performed,and moreover, after alkaline etching in the etching process, acidetching by using mixed acid containing phosphoric acid was performed.

COMPARATIVE EXAMPLE 2)

[0159] As in Example 2, etching process was performed by using lappedwafers (lapping abrasive grain size: #1200) having a diameter of 200 mm.The etching was performed through two steps of alkaline etching and acidetching by using mixed acid of hydrofluoric acid, nitric acid, andacetic acid.

[0160] As to alkaline etching, the wafers were immersed in an NaOHaqueous solution having a concentration of 50% by weight at 85° C. for450 seconds in order to perform alkali etching with a target etchingamount being set to 20 μm on both sides in total. Subsequently, afterthe wafers were immersed into a hydrogen peroxide solution of 0.3% inorder to make the surface of the wafers hydrophilic, the wafers wereimmersed in a mixed acid of hydrofluoric acid of 50% by weight:nitricacid of 70% by weight:acetic acid of 100% by weight=1:2:1 (volume ratio)having a liquid temperature of 25° C. in order to perform acid etchingwith a target etching amount being set to 10 μm on both sides in total.

[0161] The etched wafers were measured for flatness (TTV), surfaceroughness (Ra), waviness, visual inspection (and pit depth), andglossiness. The results are included in Table 6.

[0162] Next, the wafers after the above etchings were mirror-polished.The target amount of each stock removal was 4 μm. Polishing conditionswere the same as Example 2.

[0163] As for the polished wafer, each TTV and SFQRmax was measured anda visual inspection was performed. The results are included in Table 7.

[0164] As seen from the above results, in Comparative Example 2, when astock removal was set at 4 μm, pits were present thereon (Table 7:Comparative Example 2-a).

[0165] Polishing was further performed until pits were disappeared(Table 7: Comparative Example 2-b). As the result, although pits wereremoved by polishing of about 10 μm from the whole front surface, itsflatness was degraded in some degree.

EXAMPLE 3

[0166] Mirror-polished wafers were produced by the process as shown inFIG. 2. The wafers were obtained by slicing a single crystal rod (ingot)having a diameter of 200 mm and resistivity of 0.02 Ù·cm with a wiresaw, and after a first chamfering, the wafers were subjected to lappingof 40 μm on both sides in total by using lapping slurry (lappingabrasive grain size: #1200). Next, replacing the above lapping slurrywith lapping slurry of lapping abrasive grain size of #1500, the waferswere further subjected to lapping of 20 μm on both sides in total.

[0167] Subsequently, surface grinding was performed. The surfacegrinding was performed by using an in-feed type single-side grindingapparatus and using a grinding stone of #4000 at 10 μm on a single sideof the wafers. The conditions of the surface grinding were that aspindle rotational speed was 5500 rpm, a wafer rotational speed was 7rpm, and the feed rate of the grinding stone was 0.2 μm/sec. Then after,a second chamfering was performed.

[0168] At this stage, conventionally, local damages (pits) were formedon a wafer in a lapping process due to abrasive grains, but the surfacegrinding was able to be conducted with few local mechanical damages onthe wafers. Each flatness (TTV) of the wafers after this surfacegrinding was about 0.6 μm.

[0169] Next, as an etching process, first, the wafers were immersed inan NaOH aqueous solution having a concentration of 50% by weight at 85°C. in order to perform alkali etching with a target etching amount beingset to 15 μm on both sides in total. Subsequently, after the wafers wereimmersed into a hydrogen peroxide solution of 0.3% in order to make thesurface of the wafers hydrophilic, the wafers were immersed into a mixedacid of hydrofluoric acid of 50% by weight:nitric acid of 70% by weightphosphoric acid of 85% by weight=1:3:2 (volume ratio) having a liquidtemperature of 25° C. in order to perform acid etching with a targetetching amount being set to 5 μm on both sides in total.

[0170] Next, each back surface of the wafers was polished.

[0171] As to polishing conditions, polishing was performed by using apolishing apparatus, which is a single wafer processing polishingapparatus, a polishing pad, which is a nonwoven fabric type polishingpad, and a polishing agent, which is a colloidal silica polishing agent(pH=10.5). Each stock removal was 0.1 μm.

[0172] In this regard, each stock removal can also be set about 0.05-0.3μm depending on its glossiness.

[0173] Next, each chamfered portion on each periphery of the wafers wasmirror-polished.

[0174] Each front surface of the wafers in which such a process wascompleted was mirror-polished. The polishing was performed by pluralsteps (three steps of a first polishing, a second polishing, and afinish polishing). Each target stock removal amount on the whole frontsurface was 3 μm. As to main polishing conditions, polishing wasperformed by using a polishing apparatus, which is a single waferprocessing polishing apparatus, a polishing pad, which is a nonwovenfabric type polishing pad, and a polishing agent, which is a colloidalsilica polishing agent (pH=10.5).

[0175] The polished wafers were confirmed in terms of TTV and SFQRmax,glossiness of each back surface, visual inspection, and nanotopography.

[0176] Each TTV and SFQRmax were measured by using a flatness measuringdevice (U/G9500 and U/S9600) made by ADE Corporation. SFQR was measuredby using the flatness measuring device made by ADE Corporation, and thesize of each site was in an area of 20 mm×20 mm.

[0177] As to a visual inspection, the existence of a pit was confirmedwith a microscope.

[0178] If a pit was observed, its pit depth was confirmed. The pit depthwas obtained by the depth of focus of a microscope. Also, the generationof stains or the like on a wafer back surface was confirmed by a visualinspection.

[0179] Glossiness was measured with reference to JIS Z 8741 (mirrorglossiness measurement method) by the method according to that standardsby using a mirror glossiness meter (glossmeter SD) designated by thatstandards.

[0180] Nanotopography (sometimes called nanotopology) is unevenness ofwhich wavelength is about 0.1-20 mm and amplitude is about severalnanometers to 100 nanometers, and its evaluation method is that heightdifference (P-V value: Peak to Valley) on a wafer surface is estimatedin the region of a block range (called WINDOW SIZE or the like) of asquare shape about 0.1-10 mm on a side or a circular shape having adiameter of about 0.1-10 mm. This P-V value is also calledNanotopography Height or the like. As for a nanotopography, it isdesired that a maximum value of flatness present in a wafer surfaceestimated, is small. In the Example of the present invention, pluralblock areas of a square 10 mm on a side were estimated and the maximumvalue of these P-V values was evaluated. The results were shown in Table8. TABLE 8 Glossiness on TTV SFQRmax back surface Visual Nanotopography(μm) (μm) (%) Inspection (nm) Example 3 0.70 0.118 41 Nothing 50.0particular on front surface and back surface Comparative 0.78 0.160 36Stains were 103.2 Example 3 confirmed on back surface

[0181] As seen from the above results, in Example 3, a wafer having highflatness but no abnormal appearance (no grinding striation, pit or bluestain) can be produced by polishing of 4 μm or less. Particularly, sincefine unevenness at nanotopography level can improved, it is found to bepreferable.

[0182] This can be accomplished such that the surface grinding wasperformed before etching, particularly before alkaline etching, andmoreover the etching with mixed acid containing phosphoric acid wasperformed after alkaline etching in the etching process, and thepolishing on a wafer back surface was performed. Also, there were nostains or the like on the wafer back surface, and its glossiness wasgood, because it was controlled in a proper value.

COMPARATIVE EXAMPLE 3

[0183] Mirror-polished wafers were produced through the process as shownin FIG. 3. As in Example 3, wafers were obtained by slicing a singlecrystal rod (ingot) having a diameter of 200 mm and resistivity of 0.02Ù·cm by a wire saw, and after a first chamfering, the wafers weresubjected to a lapping of 40 μm on both sides in total by using alapping slurry (lapping abrasive grain size: #1200). Next, replacing thelapping slurry with a lapping slurry of lapping abrasive grain size of#1500, the wafers were further subjected to a lapping of 20 μm on bothsides in total.

[0184] Next, an etching process was performed. The etching was performedthrough two steps of an alkaline etching and an acid etching by usingmixed acid of hydrofluoric acid, nitric acid and acetic acid.

[0185] As to the alkaline etching, the wafers were immersed in an NaOHaqueous solution having a concentration of 50% by weight at 85° C. for450 seconds in order to perform alkali etching with a target etchingamount being set to 20 μm on both sides in total. Subsequently, afterthe wafers were immersed into a hydrogen peroxide solution of 0.3% inorder to make the surface of the wafers hydrophilic, the wafers wereimmersed in a mixed acid of hydrofluoric acid of 50% by weight:nitricacid of 70% by weight:acetic acid of 100% by weight=1:2:1 (volume ratio)having a liquid temperature of 25° C. in order to perform acid etchingwith a target etching amount being set to 10 μm on both sides in total.

[0186] After such a process was completed, each front surface of thewafers was mirror-polished as in Example 3. The polishing was performedby plural steps (three steps of a first polishing, a second polishing,and a finish polishing). A target stock removal amount on the wholefront surface was 3 μm. As to main polishing conditions, a polishing wasperformed by using a polishing apparatus, which is a single waferprocessing polishing apparatus, a polishing pad, which is a nonwoventype polishing pad, and a polishing agent, which is a colloidal silicapolishing agent (pH=10.5).

[0187] The polished wafers were confirmed in terms of TTV and SFQRmax,glossiness on each back surface, visual inspection, and nanotopographyas in Example 3. The results are included in Table 8.

[0188] As seen from the above results, all of TTV, SFQRmax, andnanotopography are inferior to those of Example 3. Also, since thewafers have a small resistivity, there were stains thereon.

[0189] In this regard, the present invention is not limited to theembodiment described above. The above-described embodiment is a mereexample, and those having substantially the same structure as thatdescribed in the appended claims and providing the similar functions andadvantages are included in the scope of the present invention.

[0190] For example, if etching was performed by using a lapped waferhaving a higher flatness than that used in the above Examples, a waferhaving better flatness than that produced in the above Examples can beproduced. In short, in the present invention, though the degradation offlatness due to etching can be reduced, the absolute value of TTV isalso influenced by the quality of a lapped wafer.

[0191] Also, a pit depth is also slightly influenced by a lappingabrasive grain size used in lapping. Generally, lapping abrasive grainsof about #1200 are used in a lapping process. However, its pit depth canbe further improved by using lapping abrasive grains of #1500.

[0192] Also, Examples of the present specification exemplify theconditions capable of obtaining a wafer having high flatness thatalkaline etching was performed with an etching amount of about 20 μm onboth sides in total, and acid etching was performed with an etchingamount of about 10 μm on both sides in total. However, these stockremovals are not limited thereto, and it is possible that a proportionof alkaline etching is further lowered or total etching amount isreduced. In accordance with the required quality of a wafer, theproportion of alkaline etching and acid etching is controlled, so that awafer having high flatness and shallow pit depth can be produced.

[0193] Moreover, the Examples describe the production method of a waferof which one side is highly mirror-polished, for example. However, thepresent invention is not limited thereto, and the present invention canapply to the production method of a wafer of which both sides are highlymirror-polished. As for the CW obtained through alkaline etching andacid etching containing phosphoric acid, since each pit depth of bothsides of the CW can be improved, each stock removal can be reduced whenboth sides thereof are polished, and the wafer can be processed to havehigh flatness as in the case that one side of a wafer is polished.

[0194] Particularly, the process as shown in FIG. 1(a) that a lappingprocess is omitted is preferable, and both sides of a wafer can beground by using a duplex grinding apparatus in the surface grindingprocess. After that, since the pit depth of both sides of the wafer,which is already subjected to alkaline etching and acid etchingcontaining phosphoric acid, is improved, its stock removal can bereduced when both sides thereof are polished, and the wafer can beprocessed to have high flatness as in the case that one side of a waferis polished.

[0195] Also, the above embodiment describes the case of producingsilicon wafers having a diameter of 200 mm (8 inches) in reference toExamples, for example. However, the present invention is not limitedthereto, and the present invention can apply to a silicon single crystalhaving a diameter of from 4-16 inches or more.

1. A method for processing a semiconductor wafer that the wafer obtainedby slicing a single crystal ingot is subjected to at least a chamferingprocess, a lapping process, an etching process, and a mirror-polishingprocess, wherein acid etching is performed after alkaline etching as theetching process, and the acid etching is performed with an acid etchantcomposed of hydrofluoric acid, nitric acid, phosphoric acid, and water.2. A method for processing a semiconductor wafer that the wafer obtainedby slicing a single crystal ingot is subjected to at least a chamferingprocess, a surface grinding process, an etching process, and amirror-polishing process, wherein the surface grinding process isperformed prior to the etching process, acid etching is performed afteralkaline etching as the etching process, and the acid etching isperformed with an acid etchant composed of hydrofluoric acid, nitricacid, phosphoric acid, and water.
 3. The method for processing asemiconductor wafer according to claim 2, wherein a lapping process isfurther added to the method and the processing is performed by a lappingprocess, a surface grinding process, and an etching process in order. 4.A method for processing a semiconductor wafer that the wafer obtained byslicing a single crystal ingot is subjected to at least a flatteningprocess, an etching process, and a mirror-polishing process, wherein theflattening process is performed as a previous step of the etchingprocess, acid etching is performed after alkaline etching as the etchingprocess, the acid etching is performed with an acid etchant composed ofhydrofluoric acid, nitric acid, phosphoric acid, and water, and a backsurface polishing process is performed as the mirror-polishing processafter the acid etching, and then a front surface polishing process isperformed.
 5. The method for processing a semiconductor wafer accordingto claim 4, wherein the flattening process comprises a lapping processand/or a surface grinding process.
 6. The method for processing asemiconductor wafer according to claim 4 or claim 5, wherein the backsurface polishing process is performed so that glossiness of the waferis 35-50%.
 7. The method for processing a semiconductor wafer accordingto claim 2, claim 3, claim 5, and claim 6, wherein the surface grindingprocess is performed so that a peripheral portion of the wafer becomesthicker.
 8. The method for processing a semiconductor wafer according toany one of claims 1-7, wherein a total etching amount on both sides ofthe wafer in the etching process is 30 μm or less.
 9. The method forprocessing a semiconductor wafer according to any one of claims 1-8,wherein a stock removal in the mirror-polishing process is 7 μm or less.10. The method for processing a semiconductor wafer according to any oneof claims 1-9, wherein a composition ratio of the acid etchant in mixingis that the concentration of hydrofluoric acid is 5-15% by weight andthe concentration of phosphoric acid is 10-40% by weight.
 11. The methodfor processing a semiconductor wafer according to any one of claims1-10, wherein silicon is dissolved in the acid etchant so as to have asilicon concentration of 10 g/L or more.
 12. The method for processing asemiconductor wafer according to any one of claims 1-11, wherein acomposition ratio of the acid etchant in use is that the concentrationof hydrofluoric acid is 1-7% by weight and the concentration ofphosphoric acid is 18-33% by weight.
 13. The method for processing asemiconductor wafer according to any one of claims 1-12, wherein analkaline etchant used in the alkaline etching is an NaOH aqueoussolution or a KOH aqueous solution.
 14. A semiconductor wafer, whereinthe semiconductor wafer is processed by the method according to any oneof claims 1-13.
 15. A semiconductor wafer, which is chemically etched,wherein the maximum value of a pit depth is 4 μm or less, waviness is0.05 μm or less, and glossiness is 20-70%.
 16. A semiconductor wafer ofwhich front surface is mirror-polished, wherein SFQRmax is 0.1 μm orless, the maximum value of a pit depth of the other surface, which ismirror-polished, is 4 μm or less, waviness is 0.05 μm or less, andglossiness is 20-70%.